It is known in such apparatus, or tackle, to provide means for pressing the loop of line onto the pulley to ensure that effective drive takes place without slipping of the line.
One such system is shown in DE-A-22 01 548 (TRACTEL S.A.). A guide track carrying an endless chain is fixed into the system housing near to the edge of the drive pulley. Three rollers mounted on a rocking beam are positioned inside a concave curve of the chain loop and can be moved to press the chain against the neighbouring part of the loop of line on the pulley. The rollers roll against convex inner rolling surfaces of the chain links. To ensure that the chain runs without slipping, its linking pins have end rollers which seat in circumferentially spaced notches of a drive pulley flange when pressed thereagainst, and roll on the chain track when away from the pulley.
In this system there is a good deal of rattling noise, since pressing the concave chain portion against the line shortens the chain path and hence slackens the chain around the track.
Another known system is seen in Belgian Patent 827 486. Here three pressure rollers, mounted in a housing which can be moved radially, are urgeable against the pulley periphery to engage the line directly. The possibility of having a chain loop running round the rollers is also mentioned.
In such a system with a chain, each chain link would undergo a tilting movement as it passed underneath a roller. So, instead of a uniform application of pressure there would be highly localised, non-uniform "line contacts" which could damage the line.